py3: config string params fix.

Fix set value as bytes.

Traceback (most recent call last):
  File "./services/classifier/service.py", line 28, in <module>
    from noc.fm.models.failedevent import FailedEvent
  File "/opt/noc/fm/models/failedevent.py", line 20, in <module>
    from noc.sa.models.managedobject import ManagedObject
  File "/opt/noc/sa/models/managedobject.py", line 40, in <module>
    from noc.aaa.models.user import User
  File "/opt/noc/aaa/models/user.py", line 27, in <module>
    from noc.settings import LANGUAGES
  File "/opt/noc/settings.py", line 38, in <module>
    "HOST": config.pg.addresses[0].host,
  File "/opt/noc/core/config/params.py", line 237, in __get__
    self.resolve()
  File "/opt/noc/core/config/params.py", line 251, in resolve
    if isinstance(self.services, list) and ":" in self.services[0]:
TypeError: a bytes-like object is required, not 'str'